Magnetic recording medium and method for producing the same

ABSTRACT

A magnetic recording medium comprising a substrate and, formed thereon, a magnetic layer comprised of magnetic particles bonded to each other with a thermosetting binder can be improved with respect to the durability by forming a number of fine pores in the magnetic layer and impregnating the fine pores with a lubricant. Particularly, when the fine pores have a diameter of 0.2 μm or less and a rate of the fine pore in area of 3 to 30%, the durability can be improved while maintaining the noise during the recording and reproduction of information on a low level. In preparing the magnetic recording medium having such fine pores, it is preferred that an easily thermal-decomposable additive, such as polyalkylene oxide, be added to a magnetic layer forming paint. A magnetic recording medium having further improved characteristics can be produced by interposing an undercoating layer having a number of fine pores between the magnetic layer and the substrate, allowing the fine pores in the undercoating layer to communicate with the fine pores in the magnetic layer and impregnating the fine pores with a lubricant. In this case, in forming the fine pores, it is preferable to add an easily thermal-decomposable additive to a paint or extract a thermoplastic resin from a layer containing the thermoplastic resin with a solvent.

This is a division of application Ser. No. 141,423, filed Jan. 7, 1988.

BACKGROUND OF THE INVENTION

This invention relates to a magnetic recording medium and a method forproducing the same. More particularly, the present invention isconcerned with a magnetic recording medium suitable particularly for usein applications, such as magnetic disk devices, which is excellent inwear resistance as well as in durability and reliability, and a methodfor producing the same.

The magnetic recording medium for use in magnetic devices etc. generallycomprises a substrate which may be of various types and a magnetic layerformed thereon. The surface of the magnetic recording medium oftensuffers a high-rate abrasion by a magnetic head or the like duringrecording and reproduction of information. The lowering in theperformance of the magnetic recording medium due to the above-mentionedabrasion is now a serious problem.

Therefore, it is a common practice in the art to apply a lubricant tothe surface of a magnetic layer for the purpose of protecting themagnetic layer from damages due to the abrasion with a magnetic head,etc. A preferable example of such a lubricant known in the art includesa fluorinated oil, such as perfluoropolyether, as disclosed in U.S. Pat.No. 3,778,308. A currently important task is to provide a method forsuitably utilizing this type of substance having a lubricity as alubricant for a magnetic recording medium.

When the above-mentioned lubricant is applied to a magnetic recordingmedium, such as a magnetic disk, the greater the amount of thelubricant, the more improved the durability of the magnetic recordingmedium. However, excessive application of the lubricant to the surfaceof the magnetic layer causes an increase in the adhesion force betweenthe head and the surface of the magnetic recording medium when themagnetic disk device is stopped, which raises problems that the head orthe surface of the magnetic recording medium is damaged when the deviceis restarted or, in an extreme case, the device cannot be driven at all.As mentioned above, the adhesion force between the head and the mediumis affected by the amount of the lubricant which is present on thesurface of the medium. Hitherto, various proposals have been made withrespect to a method for properly controlling the amount of the lubricantapplied onto the surface of the magnetic layer which comprises renderingthe magnetic layer porous and impregnating the porous layer with alubricant.

Examples of such proposals include a magnetic disk comprising a porousmagnetic layer composed of magnetic particles, a thermosetting resin ofepoxyphenol resin system and non-magnetic, inorganic particles, whichlayer was impregnated with a lubricant (see Japanese Patent Laid-OpenNo. 104202/1978) and a method of preparing a porous magnetic layer whichcomprises adding an organic additive, such as liquid paraffin, to amagnetic paint and thermally decomposing and evaporating the additivewhen a coating film formed is cured (see Japanese Patent Laid-Open Nos.3435/1981 and 10419/1985).

Further, in recent years, a proposal has been made on a high-densitymagnetic recording medium comprising a thermoplastic resin suitable foruse in floppy disks etc., wherein an undercoating layer containing alubricant is provided (see Japanese Patent Laid-Open No. 73235/1986).

Among the above-mentioned prior art, the method which comprisesrendering a magnetic layer porous and impregnating the porous layer witha lubricant is free from a problem with respect to the lack of thelubricant, because in the conventional magnetic recording medium havinga low recording density and a thick magnetic layer, it is possible toimpregnate the magnetic layer with a sufficient amount of a lubricant.However, with respect to a magnetic recording medium having a highrecording density for which there is an ever-increasing demand on adecrease in the thickness of the magnetic layer, it is necessary toincrease the porosity of the magnetic layer in order to impregnate athin magnetic layer with a sufficient amount of a lubricant. This leadsto a problem with respect to the durability of the magnetic recordingmedium, such as a remarkable lowering in the wear resistance of themagnetic layer.

On the other hand, the above-mentioned prior art method which comprisesadding non-magnetic particles to a magnetic coating film raises problemssuch as an increase in the noise output due to the additive. Moreover,the above-mentioned prior art method which comprises making a porousstructure through thermal decomposition of an organic additive isdisadvantageous in that a failure to choose a proper additive having adesirable compatibility with the binder of the paint leads to theformation of large pores in the coating film, thus causing noises anderrors.

Furthermore, among the above-mentioned prior art methods, the method inwhich a lubricant is incorporated in an undercoating layer provides astructure suited for a high-density magnetic recording medium having athin magnetic layer. However, in this method, the positions of the finepores of the undercoating layer do not always coincide with those of themagnetic layer and, therefore, the fine pores of the undercoating layerdo not get through to the surface of the magnetic layer, which makes itimpossible to impregnate the undercoating layer with a sufficient amountof a lubricant. Particularly, with respect to a magnetic disk comprisinga magnetic layer composed of a thermosetting resin suited for a rigiddisk, it is very difficult to impregnate the undercoating layer with alubricant, because the thermosetting resin of the magnetic layer is veryclosely hardened.

DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium having an excellent wear resistance, an improved durability and ahigh reliability, said medium being impregnated with a large amount of alubricant without spoiling the magnetic characteristics and performanceof the medium in a manner suited for a magnetic recording medium, suchas a magnetic disk, and to provide a method for producing the same.

Another object of the present invention is to provide a method forproducing a magnetic recording medium impregnated with a lubricant,which has a porous magnetic coating film having a large number of poreswhich are finer and more uniformly dispersed than those of the prior artmethods by taking advantage of an additive which does not adverselyaffect the electric characteristics of the magnetic recording mediumwhen it is employed in combination with a magnetic head.

The above-mentioned objects of the present invention can be attained byapplying a lubricant on a magnetic recording medium comprising asubstrate and, superimposed thereon in the following order, anundercoating layer having a large number of fine pores and a magneticlayer having a fine pathway for the lubricant, i.e., fine pores, whichruns from a starting point of the fine pore of the undercoating layer tothe surface of the magnetic layer.

The diameter of the above-mentioned fine pores is generally 5 μm orless, preferably 1 μm or less with respect to the undercoating layer andis 0.2 μm or less with respect to the magnetic layer. When the diameterof fine pores of the magnetic layer exceeds 0.2 μm, the noises and/orerrors caused in the recording and reproduction thereof are unfavorablyincreased. In the undercoating layer, since the size of the fine poresdoes not directly affect the occurrence of errors, the diameter of thefine pores can be increased up to an upper limit of 5 μm, which isgreater than that of the magnetic layer. When the diameter of the finepores exceeds 5 μm, however, the uniformity of the undercoating layer isdisadvantageously decreased and, therefor, the magnetic layersuperimposed thereon is indirectly affected, which unfavorably leads tothe occurrence of defects. It is to be noted that the foregoingdescription is not intended to mean that the presence of pores having adiameter larger than that mentioned above is entirely inadmissible. Inthe case of a magnetic device, it is well known that slight defects areadmissible to a certain extent.

The rate of the fine pore in area of the magnetic layer is generally 3to 30%, preferably 5 to 20%. When the rate is less than 3%, the amountof the impregnation with the lubricant is insufficient. On the otherhand, when the rate exceeds 30%, the wear resistance of the magneticlayer is unfavorably decreased. The rate of the fine pore in area of theundercoating layer is generally 5 to 60%, preferably 10 to 30%. Sincethe undercoating layer does not suffer a direct abrasion, the rate ofthe fine pore in area can be larger than that of the magnetic layer.This enables a further increase in the amount of the impregnation withthe lubricant.

In the above-mentioned range of the rate of the fine pore in area, i.e.,3 to 60%, the impregnation with a lubricant in an amount of about 2×10⁴g/m³ per % of the rate can be attained. Therefore, the ranges from 3 to30%, from 5 to 20%, from 5 to 60%, and from 10 to 30% as mentioned abovewith respect to the rate of the fine pore in area correspond to theamounts of the impregnation with a lubricant from 6×10⁴ to 60×10⁴ g/m³,from 10×10⁴ to 40×10⁴ g/m³, from 10×10⁴ to 120×10⁴ g/m³, and from 20×10⁴to 60×10⁴ g/m³, respectively. These amounts of impregnation with thelubricant were calculated using a representative specific gravity of 2which is close to the specific gravity of the lubricant generallyapplied to a magnetic disk, i.e., perfluoroalkylpolyether. Theterminology "rate of the fine pore in area" as used in the presentspecification is intended to mean the total area of fine pores per unitarea as viewed in a photograph or the like taken with a scanningelectron microscope (SEM).

A first method of producing the magnetic recording medium according tothe present invention comprises coating a substrate with a paintcomprising a thermosetting resin, an easily thermal-decomposableadditive having thermal decomposability greater than that of thethermosetting resin and a solvent, thereby forming an undercoatinglayer, and conducting primary curing of the layer at a such atemperature that the above-mentioned additive can survive. Subsequently,a magnetic layer forming paint comprising magnetic particles, a bindercomposed mainly of a thermosetting resin and a solvent is applied on theundercoating layer, followed by curing (second curing) of the resultingmagnetic layer and the undercoating layer at a temperature higher thanthe primary curing temperature of the undercoating layer. During thesecondary curing, the easily thermal-decomposable additive in theundercoating layer is thermally decomposed and scattered. As a result,fine pores are formed in the undercoating layer at the places wherethermal decomposition of the additive has occurred, and a pathwaystarting from the fine pores through which a thermal decompositionproduct of the additive has been scattered serves as a fine pathway forthe lubricant in the magnetic layer, thereby obtaining a magneticrecording medium before impregnation with a lubricant according to thepresent invention.

When fine pores running from the undercoating layer to the surface ofthe magnetic layer are formed by the thermal decomposition andscattering of the easily thermal-decomposable additive according to theabove-mentioned method, the thermal decomposability characteristics ofthe easily thermal-decomposable additive and the compatibility of thebinder of the undercoating layer composed mainly of a thermosettingresin with the additive are important. With respect to the selection ofthe easily thermal-decomposable additive, the following considerationmust be taken into.

Specifically, since the easily thermal-decomposable additive is a sourcefor formation of fine pores, the additive must remain in a uniformlydispersed state in the undercoating layer without causing any phaseseparation which is causative of a defect of the undercoating, i.e., aphase separation of a size greater than the thickness of theundercoating layer, even after evaporation of the solvent of theundercoating layer applied onto a substrate. Therefore, it is necessarythat the additive have some compatibility with the binder of theundercoating layer. When the easily thermal-decomposable additivemeeting the above requirements is employed, an even undercoating layeris formed, which is suited as a primary coat layer for a magneticrecording layer.

The compatibility of the easily thermal-decomposable additive will nowbe described in more detail.

When the solubility of the easily thermal-decomposable additive in amagnetic paint (the additive can be added also to a magnetic paint aswill be described later) or in an undercoating layer is low, the numberof fine pores is unfavorably decreased. Accordingly, the solubility ofthe easily thermal-decomposable additive in the paint is generally notlower than 5% by weight, preferably not lower than 20% by weight. Thereis no particular upper limit for the solubility. The most criticalfactor affecting the solubility of the easily thermal-decomposableadditive in the paint is a solvent for the paint.

When a coating film is formed through evaporation of the solvent, thecompatibility between the binder and the easily thermal-decomposableadditive is important. This compatibility can be evaluated in terms ofthe diameter of fine pores. As mentioned before, the easilythermal-decomposable additive has a compatibility with the binder of theundercoating layer such that the undercoating layer has pores ofgenerally 5 μm or less, preferably 1 μm or less, and a compatibilitywith the binder of the magnetic layer such that the magnetic layer haspores of 0.2 μm or less. When the compatibility is excellent, thediameter of the fine pores is small. On the other hand, when thecompatibility is poor, the diameter of the fine pores is large.

The requirements for the thermal decomposition characteristics of theeasily thermal-decomposable additive will now be described. When amagnetic layer is formed by applying a magnetic layer forming paint onan undercoating layer, it is necessary to preliminary conduct curing(primary curing) of the undercoating layer to a certain extent so thatthe layer is not adversely affected by the solvent of the paint. It isnoted in this connection that the additive must be chosen from thosecapable of sufficiently surviving at the temperature of the primarycuring in order to ensure an effective action of the additive. Moreover,in order to form fine pores reaching the surface of the magnetic layerby the thermal decomposition and scattering of the easilythermal-decomposable additive, it is necessary for a thermaldecomposition product to occur from the additive during the curing ofthe magnetic layer and before the completion thereof. A typical exampleof the thermosetting resin employed as a binder for the magnetic layerof a magnetic disk or the like is an epoxy resin (a mixture of an epoxyresin, a phenolic resin, a vinyl resin, etc.). For example, when such athermosetting resin is employed as a binder for the undercoating layer,curing of the binder is generally conducted at a temperature from about170° to 250° C. Accordingly, in this case, the easilythermal-decomposable additive to be employed is chosen from thecompounds which can survive without undergoing thermal decomposition ata lower temperature range such as that indicated above but are mostlythermally decomposed and scattered at a higher temperature range. Apreferable substance meeting both of the above-mentioned compatibilityand thermal decomposability requirements is an organic polymer having acertain compatibility with the binder. Examples of such an organicpolymer include a poly(alkylene oxide). The poly(alkylene oxide) iscompatible with an epoxy phenolic binder. Further, the polyalkyleneoxide having a suitable molecular weight can survive by 80% or more at alower temperature range, i.e., about 170° C. and can be thermallydecomposed and scattered by 90% or more at a high temperature range,i.e., 220° to 250° C. Examples of the poly(alkylene oxide) includepoly(butene oxide), poly(propylene oxide) and a copolymer thereof and acopolymer of ethylene oxide with propylene oxide.

A method for forming a magnetic recording medium according to thepresent invention has been described above with reference to the casewhere a thermosetting resin is used as a binder of the undercoatinglayer. However, the binder of the undercoating layer may be a resincurable with ultraviolet rays or electron beams, e.g., epoxy-acrylateresin system. In this case, the primary curing of the undercoating layeris conducted by irradiation with ultraviolet rays or electron beams,which makes it possible to cure the undercoating layer at a lowtemperature, thus leading to an advantage such that the loss of theeasily thermal-decomposable additive at a primary curing temperature isnegligible.

It is preferred that the following matter be taken into consideration informing the magnetic recording medium of the present invention. First,with respect to the thickness of the undercoating layer, the larger thethickness, the larger the amount of impregnation with a lubricant suchas perfluoroalkylpolyether. The thickness of the undercoating layer ispreferably 0.5 time or more the thickness of the magnetic layer. It isnoted in this connection that a thickness of 5 μm or less is preferableto obtain an even coating. With respect to the thickness of the magneticlayer, the smaller the thickness, the higher the density of magneticrecording. In the magnetic recording medium of the present invention, asuitable thickness of the magnetic layer is 0.1 to 0.8 μm. Since theshape of the surface of the undercoating layer affects the occurrence ofdefects of the magnetic recording medium, noises, etc., it is preferredthat, if necessary, surface finishing be conducted after the primarycuring of the undercoating layer to make the surface even. Further, inorder to attain a useful effect of the easily thermal-decomposableadditive contained in the undercoating layer, it is preferable tosuppress the shrinkage on curing of the undercoating layer during thethermal decomposition and scattering of the thermal-decomposableadditive. An effective expedient for attaining this purpose is toincorporate non-magnetic particles, such as α-Fe₂ O₃, SiO₂, Al₂ O₃, SiC,ZrO₂, or polymer particles, in the undercoating layer. The amount ofaddition of the non-magnetic particles is preferably 65% by volume orless based on the binder of the undercoating layer (total amount of thebinder and the particles: 100% by volume). Magnetic particles, such asγ-Fe₂ O₃, iron powder, γ-Fe₂ O₃ containing cobalt, Fe₃ O₄, Fe₃ O₄containing cobalt, or barium ferrite, may be also added in order toimprove the magnetic characteristics of the magnetic recording mediumthrough the impartment of the capacity for impregnation with a lubricantand magnetism to the undercoating layer. The amount of addition of themagnetic particles is preferably 65% by volume or less based on thebinder of the undercoating layer (total amount of the binder and theparticles: 100% by volume).

The preferable diameter of the above-mentioned non-magnetic particles isless than twice the thickness of the under coating layer. It isunfavorable for the particle diameter of the non-magnetic particlescontained in the undercoating layer to exceed this value, because thereoccurs defects when the particles are protruded into the magnetic layer.Although there is no particular restriction with respect to the lowerlimit of the particle diameter, the particle diameter of thenon-magnetic particles obtained by current techniques is generally 0.02μm or more.

Since the above-mentioned magnetic particles incorporated in theundercoating layer is related to the magnetic characteristics of themagnetic recording medium, it is preferred that the particle diameter ofthe magnetic particles be substantially the same as that of the magneticparticles used in the magnetic film of an ordinary recording medium,i.e., 0.5 μm to 0.02 μm.

In the magnetic layer applied on the undercoating layer, a fine pathwayfor a lubricant is formed through the thermal decomposition andscattering of the easily thermal-decomposable additive contained in theundercoating layer even when it is free from any thermal-decomposableadditive. However, the addition of the thermal-decomposable additive toa magnetic layer forming paint leads to more effective formation of thefine pathway. In this case, the thermal-decomposable additive added tothe undercoating layer may different from that added to the magneticlayer forming paint.

The amount of addition of the easily thermal-decomposable additive tothe undercoating layer is 5 to 60% by weight based on the binder (totalamount of the binder and the additive: 100% by weight), preferably 10 to50% by weight. When the amount of addition of the thermal-decomposableadditive is below the above-mentioned range, any sufficient impregnationwith the lubricant cannot be attained. On the other hand, when theamount of addition exceeds the above-mentioned range, the wearresistance of the undercoating layer is lowered, which unfavorably makesit difficult to conduct surface finishing.

When an easily thermal-decomposable additive is added to the magneticlayer forming paint, the amount of addition of the thermal-decomposableadditive is 30% by weight or less based on the binder (the total amountof the binder and the additive: 100% by weight), preferably 5 to 30% byweight. The addition in an amount exceeding 30% by weight unfavorablybrings about not only an increase in noises during the recording andreproduction of information with respect to the magnetic recordingmedium but also a lowering in the wear resistance. An apparent effectderived from the addition of the thermal-decomposable additive to themagnetic paint can be attained when the amount of addition is 5% byweight or more.

It is needless to say that, in the magnetic recording medium of thepresent invention, the binder for the thermosetting resin used in theformation of the magnetic layer may be the same as or different from thebinder used in the formation of the undercoating layer.

A second method for producing the magnetic recording medium according tothe present invention comprises coating a substrate with a paintcontaining a binder, such as the same thermosetting resin as that usedin the above-mentioned first method, an ultraviolet-curable resin and anelectron beam-curable resin, a thermoplastic resin compatible with saidbinder, and a solvent to thereby form an undercoating layer, subjectingsaid undercoating layer to primary curing by heating, ultravioletradiation or electron beam radiation at such a temperature that saidthermoplastic resin can survive without undergoing evaporation orthermal decomposition, coating said undercoating layer which has beensubjected to primary curing with a magnetic layer forming paintcontaining magnetic particles, a thermosetting resin and a solvent,thereby forming a magnetic layer, subjecting said magnetic layer andsaid undercoating layer to secondary curing at such a temperature thatsaid thermosetting resin of said magnetic layer is incompletely curedand said thermoplastic resin contained in said undercoating layer cansurvive, immersing said substrate having said incompletely curedundercoating and said magnetic layer formed thereon in a solvent capableof dissolving said thermoplastic resin contained in said undercoatinglayer to extract and remove said thermoplastic resin from saidundercoating layer through said incompletely cured magnetic layer, andsubjecting said undercoating layer and said magnetic layer to tertiarycuring at such a temperature that said heat curable binder contained insaid undercoating layer and said magnetic layer is completely cured,thereby forming fine pores in said undercoating layer and, at the sametime, forming a fine pathway in said undercoating layer which reachesthe surface of said magnetic layer. In the above-mentioned second methodaccording to the present invention, an alcoholic solvent, such as methylalcohol, or an ethereal solvent, such as ether, may be used as thesolvent for removing the thermoplastic resin from the undercoatinglayer. Further, in the second method according to the present invention,the thermoplastic resin used in the undercoating layer may be the samesubstance as the easily thermal decomposable additive used in theabove-mentioned first method according to the present invention andincludes poly(alkylene oxide), and other conditions of production may besubstantially the same as those employed in the first method accordingto the present invention.

In this second method as well, the fine pathway can be more effectivelyformed by the addition of a fine pore forming resin, i.e., athermoplastic resin in this case, to the magnetic resin as mentionedwith respect to the first method.

The amount of the thermoplastic resin to be added to the undercoatinglayer and the magnetic layer forming paint is the same as that ofaddition of the easily thermal decomposable additive in theabove-mentioned first method. In both the first and second methods, anymagnetic paint known in the art may be used, except that an easilythermal-decomposable additive or a thermoplastic resin is added theretoaccording to need.

Further, the second method is the same as the first method, except thata thermoplastic resin is added instead of the thermal-decomposableadditive and the thermoplastic resin is extracted and removed when thethermosetting resin is in an incompletely cured state.

In both the first and second methods, the magnetic recording medium ofthe present invention can be obtained by impregnating a dual layercomprising the magnetic layer and the undercoating layer with alubricant from the surface of the magnetic layer after the formation ofthe magnetic layer. As mentioned above, a fluorinated oil, such asperfluoropolyether, is preferably used as the lubricant.

In the magnetic recording medium of the present invention, a number offine pores constituting a fine pathway which runs from the fine pores ofthe undercoating layer as the starting point to the surface of themagnetic layer are formed within the undercoating layer. Therefore, theapplication of the lubricant on the surface of the magnetic layer causesan impregnation of the undercoating with a large amount of lubricant,which enables the formation of a magnetic recording medium having wearresistance and durability superior to the conventional magneticrecording medium.

The above-mentioned poly(alkylene oxide) exhibits an excellent effectwhen it is used in forming pores in a magnetic layer containing athermosetting resin as a binder. Therefore, in the production of amagnetic recording medium in which a magnetic layer is directly providedon a substrate free from the undercoating layer, a lubricant-impregnatedmagnetic recording medium provided with a porous magnetic coating filmhaving fine pores which are finer and more uniformly dispersed than thefine pores in the conventional magnetic recording medium can be obtainedby adding a poly(alkylene oxide) to the magnetic paint and thermallydecomposing the polyalkylene oxide during the curing of the magneticcoating film to form a number of fine pores in the magnetic coatingfilm. These fine pores reach the surface of the magnetic coating film.The magnetic recording medium thus obtained can be impregnated with alubricant in an amount larger than that of impregnation with a lubricantin the case of the conventional lubricant-impregnated magnetic recordingmedium without causing any increase in noises and errors. The method forproducing this kind of magnetic recording medium will now be described.

This magnetic recording medium can be produced by a method whichcomprises adding a poly(alkylene oxide) which is a polymer compatiblewith a binder and having a thermal decomposition temperature which issubstantially the same as or lower than the curing temperature of thebinder and forming a coating on a substrate by making use of themagnetic paint and heat treating the coating to cure the binder and, atthe same time, to thermally decompose and evaporate the poly(alkyleneoxide).

As mentioned above, when fine pores are formed in the magnetic filmthrough the thermal decomposition and evaporation of the additive, thethermal decomposability of the additive and the compatibility of theadditive with the binder are critical.

The poly(alkylene oxide) which is an additive used in the presentinvention is a polymer having an ether linkage in its main chain. Thethermogravimetric analysis (TGA) revealed that this substance has athermal decomposition temperature higher than that of a substance freefrom an ether linkage, e.g., liquid paraffin, polyethylene, orpolybutene, and causes rapid thermal decomposition at about 180° to 230°C.

Further, the poly(alkylene oxide) exhibits excellent compatibility withthe binder of the magnetic paint. For example, a binder composed mainlyof an epoxy resin or a phenolic resin (50% by weight or more) is widelyused as a heat-curable binder for a magnetic coating film of a magneticdisk or the like. The poly(alkylene oxide) exhibits excellentcompatibility with these resins. In this case, the compatibility variesdepending upon the proportion of the enter linkage in the poly(alkyleneoxide). Polymers of propylene oxide, butene oxide, and heptene oxide, orcopolymers comprising these polymers as main component (30% by weight ormore) and other alkylene oxides exhibit particularly excellentcompatibility.

Since the additive according to the present invention has excellentcompatibility with the paint, it can be added in a wide range ofconcentration. However, when the amount of addition is too large, thediameter of the pores is large, which brings about the occurrence ofnoises. On the other hand, when the amount of addition is too small,fine pores in an amount sufficient for impregnation with a lubricantcannot be formed. For this reason, a suitable amount of addition is 1 to30% by weight based on the binder of the magnetic paint.

The method for producing a magnetic recording medium according to thepresent invention may be the same as the conventional method, exceptthat, as mentioned above, 1 to 30% by weight, based on the binder of themagnetic paint, of a poly(alkylene oxide) is added to the magneticpaint. However, for better understanding of the present invention, themethod for producing a magnetic recording medium according to thepresent invention will be further described in comparison with the priorart.

Any thermosetting binder known in the art can be used in the presentinvention. Examples of such a thermosetting binder include an epoxyresin, a phenolic resin, a vinyl resin, a polyester, a cellulosederivative, a melamine resin, a polyurethane, a polyamide, an acrylicresin, a methacrylic resin or their copolymers. They may be used aloneor in the form of any mixture thereof. A particularly preferablethermosetting resin is a mixture composed mainly of an epoxy resin or aphenolic resin or a mixture of both resins and other resin addedthereto, such as a vinyl resin, an acrylic resin, a methacrylic resin,or a polyester.

It is preferred that the magnetic powder be used in an amount of 15 to65% by volume. When the magnetic powder is γ-Fe₂ O₃ (specific gravity:5), the above-mentioned amount corresponds to about 50 to 90% by weight.

If necessary, a reinforcing agent may be incorporated in the magneticcoating film. α-Al₂ O₃ powder is ordinarily used as the reinforcingagent. Besides this, hard inorganic substances, such as SiC or ZrO₂, maybe also used as the reinforcing agent.

It is necessary that the magnetic coating film be cured so as to havedurability sufficient to be used as a magnetic recording medium. Forthis reason, it is a matter of course that the heat treatment in thefinal step is conducted in the same manner as that of the conventionalmethod, i.e., at such a temperature that the binder is sufficientlycured. As mentioned above, the poly(alkylene oxide) as an additive usedin the present invention exhibits thermal decomposability which is inagreement with the curing temperature of the epoxy-phenolic resin, i.e.,170° to 250° C., which gives an excellent result in the above-mentionedheat treatment.

It is preferred that the thermal decomposition temperature of thepoly(alkylene oxide) be substantially the same as or lower than thecuring temperature of the thermosetting binder. When the additive islost through thermal decomposition or evaporation in the step oftemperature elevation (a temperature of 170° C. or below in the case ofan epoxy-phenolic binder) before the initiation of the curing of thebinder, formed pores disappear in the subsequent step of curing, whichmakes it impossible to form a porous coating film. Further, no porouscoating film can be obtained when the additive does not disappearthrough thermal decomposition at a curing temperature of the coatingfilm (220° to 250° C. in the case of an epoxy-phenolic binder).Therefore, it is desirable to use as the additive a polymer which cansurvive by 80% or more when it is heated at a temperature of about 170°C. at which the binder begins to cure and causes substantially completethermal decomposition (90% or more) at a curing temperature of thebinder, i.e., 220° to 250° C. The poly(alkylene oxide) meets thisrequirement and favorably causes thermal decomposition when the thermalcuring proceeds to a certain extent.

After the completion of the thermal curing treatment of the magneticlayer, the magnetic layer is impregnated with a lubricant, e.g.perfluoropolyether, from the surface of the magnetic layer, therebyobtaining the magnetic recording medium of the present invention.

It is noted that the heat treatment of the magnetic layer may beconducted by a two-stage heat treatment method which comprises conducingprimary curing of the magnetic coating film at about 180° C. which doesnot cause thermal decomposition of the poly(alkylene oxide) and thenheat treating the magnetic layer at an ordinary curing temperature,i.e., about 230° C., thereby completely curing the magnetic layer and,at the same time, thermally decomposing the additive. However, asatisfactory result can be usually obtained by simply conducing thethermal decomposition at about 230° C.

The additive for the magnetic paint used in the method for producing amagnetic recording medium according to the present invention is asubstance selected while sufficiently taking into consideration thecompatibility with binder, thermal decomposability, etc. The formationof a coating by making use of this substance and a subsequent suitableheat treatment of the coating brings about the formation of a number offine pores having a pore diameter as small as 0.2 μm or less, which wasimpossible in the prior art methods, which makes it possible to obtain amagnetic coating film impregnated with a large amount of a lubricant. Inthis case, the rate of the fine pore in area is 3 to 30%, preferably 5to 20%. This enables the realization of a magnetic recording mediumexhibiting reduced noises during the recording and reproduction ofinformation and excellent durability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views of typical examples of themagnetic recording medium of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

Magnetic disks in the present example and comparative examples whichwill be described later were prepared and evaluated by the followingprocedures.

3000 parts by weight of a solvent was added to a mixture comprising 600parts by weight of magnetic powder (γ-Fe₂ O₃), 20 parts by weight of areinforcing agent (α-Al₂ O₃) and 380 parts by weight of an epoxyphenolicbinder [a mixture of 44% (by weight or by volume with similar results;the same shall apply hereinafter) of an epoxy resin, 44% of a phenolicresin, and 12% of a vinyl resin], followed by kneading with a kneader ora ball mill to obtain a magnetic layer forming paint (magnetic paint).An easily thermal-decomposable additive was added during kneading withthe kneader or ball mill, or after the preparation of the magneticpaint. The solvent is a cyclohexanone/ isophorone solvent prepared bymixing cyclohexanone, isophorone, and dioxane at a ratio of 6:3:1. Amixed solvent comprising cellosolve acetate and butyl cellosolve may beused as the solvent.

Undercoating layer forming paints (undercoat paint) were prepared asfollows. An undercoating layer forming paint free from non-magneticparticles was prepared by adding 750 parts by weight of the same solventas the one used above to 250 parts by weight of the same epoxy-phenolicbinder as the binder of the magnetic paint, followed by mixing with anagitator. Further, an undercoating layer forming paint containingnon-magnetic particles (α-Fe₂ O₃) in the form of needle-like powderhaving a major axis of 0.4 μm and a minor axis of 0.05 μm was preparedby adding 2500 parts by weight of a solvent to a mixture of 500 parts byweight of the non-magnetic particles and 500 parts by weight of anepoxy-phenolic binder, followed by kneading with a kneader or a ballmill. An easily thermal-decomposable additive was added after thecompletion of the preparation of the undercoating paint, followed byagitation.

Magnetic disks each having a cross section as shown in FIG. 1 wereprepared by making use of the paints thus obtained according to thefollowing procedures. First, the paint for an undercoating layer wasapplied on a substrate 3 made of aluminum, followed by drying. Thecoating film of the undercoating layer thus formed was cured at 180° C.for 2 hr (primary curing) and then finished so as to have predeterminedsurface roughness and thickness. Then, the magnetic paint was applied onthe undercoating layer 2, followed by drying. The magnetic coating filmthus formed was cured together with the undercoating layer at 230° C.for 2 hr (secondary curing). Thereafter, the surface of the magneticcoating film was ground and finished to form a magnetic layer having apredetermined thickness. A predetermined amount of a lubricant(perfluoroalkylpolyether; a product of E. I. du Pont de Nemours & Co.sold under the trade name of Krytox) was applied on the surface of themagnetic layer to prepare a magnetic disk.

A magnetic disk having an undercoating layer made of anultraviolet-curable resin was prepared in the same manner as that in thecase where a thermosetting resin was used, except that, instead offorming the undercoating layer 2 with the thermosetting resin, theundercoating layer 2 was formed by making use of an undercoat paintprepared by the same method as that described above with the exceptionthat an epoxy-acrylate resin was used as the epoxy-phenolic resininstead of the epoxy resin and that the undercoating layer was cured byirradiation with ultraviolet rays using a metal halide lamp in anitrogen atmosphere for 5 min instead of curing at 180° C. for 2 hr(primary curing).

Further, a magnetic disk having an undercoating layer made of anelectron beam-curable resin was prepared by forming an undercoatinglayer by making use of an epoxy-acrylate resin as an electronbeam-curable resin instead of forming the undercoating layer 2 with thethermosetting resin and exposing the undercoating layer to a shower ofelectron beams to which a voltage of 200 kV had been applied.

The magnetic disks as prepared above each contained magnetic particles 5in the magnetic layer 1 and had a fine pore 6 in the undercoating layer2 and a fine pathway 7 which ran from the fine pore 6 to the surface ofthe magnetic layer.

The adhesion force between a magnetic head and the disk was evaluated interms of the force applied to the head when the magnetic head was put onthe surface of the disk and the disk was rotated. This adhesion forcerapidly increased when the amount of the lubricant exceeded a certainvalue. This critical value was regarded as the amount of impregnationwith the lubricant.

The test results in the present example are summarized in Table 1. Ascan be seen from Table 1, the magnetic disks formed by adding an easilythermal-decomposable additive to the undercoating layer, i.e., examplesample Nos. 1 to 8, is apparently superior in the amount of impregnationof lubricant to the magnetic disk free from the undercoating layer(comparative example sample No. 1) or the disk which had been formedwithout addition of the thermal-decomposable additive to theundercoating layer (comparative example sample No. 2). Further, thefollowing facts can be observed in Table 1. The amount of impregnationwith a lubricant is increased by the addition of non-magnetic particlesor magnetic particles to the undercoating layer (example sample Nos. 2and 3). The amount of impregnation with a lubricant is further increasedby addition of an easily thermal-decomposable additive to the magneticpaint (example sample No. 4). The thicker the undercoating layer, thelarger the amount of impregnation with a lubricant (example sample No.6). The same effect can be attained even when an ultraviolet-curableresin or an electron beam-curable resin is used in the undercoatinglayer (example sample Nos. 7 and 8).

The observations of the samples under a scanning electron microscoperevealed that all of the samples obtained in the present example had anumber of fine pores on the surface of the undercoating layer, said finepores having a diameter of 5 μm or less and a rate of the fine pore inarea of 5% or more and communicating with a fine pathway in the magneticlayer.

                                      TABLE 1    __________________________________________________________________________            undercoating layer magnetic layer                        thermal-    thermal-                                           amount of                 added  decomposable                                    decomposable                                           impregnation            coating                 particles                        additive                               coating                                    additive                                           with    sample  thickness                 (amount of                        (amount of                               thickness                                    (amount of                                           lubricant    No.     (μm)                 addition)                        addition)                               (μm)                                    addition)                                           (mg/m.sup.2)                                                  remarks    __________________________________________________________________________    Ex. 1   0.5  --     polybutene                               0.3  --     162                        oxide                        (10 pts. wt.)        2   0.5  αFe.sub.2 O.sub.3                        polybutene                               0.3  --     185                 (100 pts. wt.)                        oxide                        (10 pts. wt.)        3   0.5  αFe.sub.2 O.sub.3                        polybutene                               0.3  --     175                 (50 pts. wt.)                        oxide                 γFe.sub.2 O.sub.3                        (10 pts. wt.)                 (50 pts. wt.)        4   0.5  αFe.sub.2 O.sub.3                        polybutene                               0.3  polybutene                                           227                 (100 pts. wt.)                        oxide       oxide                        (10 pts. wt.)                                    (5 pts. wt.)        5   0.3  αFe.sub. 2 O.sub.3                        polybutene                               0.3  polybutene                                           168                 (100 pts. wt.)                        oxide       oxide                        (10 pts. wt.)                                    (5 pts. wt.)        6   1.0  αFe.sub.2 O.sub.3                        polybutene                               0.3  polybutene                                           306                 (100 pts. wt.)                        oxide       oxide                        (10 pts. wt.)                                    (5 pts. wt.)        7   1.0  --     polybutene                               0.3  --     158    ultraviolet-                        oxide                     cured                        (10 pts. wt.)             undercoating                                                  layer        8   1.0  --     polybutene                               0.3  --     150    electron                        oxide                     beam-cured                        (10 pts. wt.)             undercoating                                                  layer    Comp.        1        --     --     0.3  --      38    Ex. 2   0.5  --     --     0.3  polybutene                                            85                                    oxide                                    (5 pts. wt.)    __________________________________________________________________________

In Table 1, the amounts of addition of the added particles and thethermal-decomposable additive were expressed by parts by weight based on100 parts by weight of the binder of the undercoating layer or themagnetic layer. "TERATHANE 2900" (the trade name of a product of E.I. duPont de Nemours & Co.) was used as polybutene oxide. The amount ofimpregnation with a lubricant was expressed in terms of the amount ofdeposition of the lubricant (mg) per unit surface of the magnetic disk(m²).

Example 2

In the present example, a magnetic paint and an undercoat paint wereprepared in substantially the same manner as in Example 1, except that acopolymer of ethylene oxide with propylene oxide was used instead of thethermal-decomposable additive as the thermoplastic resin added to theundercoating layer. Thereafter, a magnetic disk was prepared accordingto the following procedures. First, the paint for forming anundercoating layer was applied on a substrate made of aluminum, followedby drying. The coating film thus formed was cured at 180° C. for 2 hr(primary curing) and then finished so as to have predetermined surfaceroughness and thickness. Then, the magnetic paint was applied on theundercoating layer, followed by drying. The magnetic coating film thusformed was cured together with the undercoating layer at 180° C. for 2hr (secondary curing). Thereafter, the resulting disk was immersed inmethyl alcohol for 2 hr or longer, thereby extracting and removing thethermoplastic resin contained in the undercoating layer.

Then, the disk was subjected to curing at 230° C. for 2 hr. The surfaceof the magnetic coating film of the magnetic layer was ground andfinished so as to have a predetermined thickness. A predetermined amountof a lubricant (perfluoroalkylpolyether; a product of E.I. du Pont deNemours & Co., sold under the trade name of Krytox) was applied on thesurface of the magnetic layer to prepare a magnetic disk. The amount ofimpregnation with a lubricant was evaluated in the same manner as inExample 1.

The test results in the present example are shown in Table 2.

                                      TABLE 2    __________________________________________________________________________            undercoating layer magnetic layer                                           amount of                 added  thermoplastic                                    thermoplastic                                           impregnation            coating                 particles                        resin  coating                                    resin  with    sample  thickness                 (amount of                        (amount of                               thickness                                    (amount of                                           lubricant    No.     (μm)                 addition)                        addition)                               (μm)                                    addition)                                           (mg/m.sup.2)                                                  remarks    __________________________________________________________________________    Ex.  9  0.5  --     copolymer of                               0.3  --     176                        ethylene                        oxide with                        propylene                        oxide                        (15 pts. wt.)        10  0.5  αFe.sub.2 O.sub.3                        copolymer of                               0.3  --     192                 (100 pts. wt.)                        ethylene                        oxide with                        propylene                        oxide                        (15 pts. wt.)        11  0.5  αFe.sub.2 O.sub.3                        copolymer of                               0.3  --     185                 (50 pts. wt.)                        ethylene                 γFe.sub.2 O.sub.3                        oxide with                 (50 pts. wt.)                        propylene                        oxide                        (15 pts. wt.)        12  0.5  αFe.sub.2 O.sub.3                        copolymer of                               0.3  copolymer of                                           256                 (100 pts. wt.)                        ethylene    ethylene                        oxide with  oxide with                        propylene   propylene                        oxide       oxide                        (15 pts. wt.)                                    (10 pts. wt.)        13  0.3  αFe.sub.2 O.sub.3                        copolymer of                               0.3  copolymer of                                           188                 (100 pts. wt.)                        ethylene    ethylene                        oxide with  oxide with                        propylene   propylene                        oxide       oxide                        (15 pts. wt.)                                    (10 pts. wt.)        14  1.0  αFe.sub.2 O.sub.3                        copolymer of                               0.3  copolymer of                                           298                 (100 pts. wt.)                        ethylene    ethylene                        oxide with  oxide with                        propylene   propylene                        oxide       oxide                        (15 pts. wt.)                                    (10 pts. wt.)        15  0.5  --     copolymer of                               0.3  --     172    ultraviolet-                        ethylene                  cured                        oxide with                undercoating                        propylene                 layer                        oxide                        (15 pts. wt.)        16  0.5  --     copolymer of                               0.3  --     178    electron                        ethylene                  beam-cured                        oxide with                undercoating                        propylene                 layer                        oxide                        (15 pts. wt.)    Comp.         3  0    --     --     0.3  --      38    Ex.  4  0.5  --     --     0.3  copolymer of                                            85                                    ethylene                                    oxide with                                    propylene                                    oxide                                    (5 pts. wt.)    __________________________________________________________________________

In Table 2, the amounts of addition of the added particles and thethermal-decomposable additive were expressed by parts by weight based on100 parts by weight of the binder of the undercoating layer and themagnetic layer. "PLURONIC L121" (the trade name of a product of AsahiDenka Kogyo K.K.) was used as the copolymer of ethylene oxide withpropylene oxide.

As can be seen from Table 2, the magnetic disks formed by adding athermoplastic resin to the undercoating layer, i.e., example sample Nos.9 to 16, is apparently superior in the amount of impregnation with alubricant to the magnetic disk free from the undercoating layer(comparative example sample No. 3) or the disk which had been formedwithout addition of any thermoplastic resin to the undercoating layer(comparative example sample No. 4). Further, the following facts can beobserved in Table 1. The amount of impregnation with a lubricant isincreased by addition of non-magnetic particles or magnetic particles tothe undercoating layer (example sample Nos. 10 and 11). The amount ofimpregnation with a lubricant is further increased by addition of athermoplastic resin to the magnetic paint (example sample No. 12). Thethicker the undercoating layer, the larger the amount of impregnationwith a lubricant (example sample No. 14).

The same fine pores as those obtained in Example 1 were observed in thisexample.

Example 3

A magnetic paint composed of magnetic powder, a reinforcing agent, abinder of expoxy-phenol resin system, a solvent and an additive such asan easily thermal-decomposable additive was prepared in the same manneras in Example 1.

The magnetic paint thus prepared was applied on a substrate made ofaluminum by spin coating to form a magnetic coating film. The magneticcoating film was dried and finally completely cured, followed by curingat such a temperature that the additive can be thermally decomposed.Thereafter, the surface of the magnetic coating film was ground andfinished to have a thickness of about 0.5 μm. A predetermined amount ofa lubricant (perfluoroalkylpolyether; a product of E.I. du Pont deNemours & Co., sold under the trade name of Krytox) was applied on thesurface of the magnetic layer to prepare a magnetic disk. Across-sectional view of this magnetic disk is shown in FIG. 2. In FIGS.1 and 2, like portions are designated by like numerals.

The amount of impregnation with a lubricant of this magnetic disk wasdetermined by measuring the adhesion force between the magnetic head andthe magnetic disk in the same manner as in Example 1.

The test results in the present example are shown in Table 3. As can beseen from Table 3, the magnetic disks formed by adding thethermal-decomposable additive to the undercoating layer were apparentlysuperior in the amount of impregnation with a lubricant to the magneticdisk which had been formed without addition of any thermal-decomposableadditive, i.e. Comparative Example 5. Particularly, as is apparent fromthe results with respect to example sample Nos. 17 to 20, the magneticdisks formed by adding an polyalkylene oxide exhibited a remarkablyincreased amount of impregnation with a lubricant. The surface of eachof the magnetic disks was observed under a scanning electron microscope.As a result, it was found that the magnetic disks of comparative examplesample Nos. 6 and 7 had a number of pores having a diameter of about 0.4μm or more, which led to a high reproduction noise output. On the otherhand, the magnetic disks of example sample Nos. 17 to 20 each had aporous magnetic coating film comprising fine pores having a diameter ofabout 0.2 μm or less and exhibited a reproduction noise outputcomparable to that of the magnetic disk of comparative example No. 5formed without addition of any thermal-decomposable

                                      TABLE 3    __________________________________________________________________________    sample                        amount of    No. of                 amount of                                  impregnation    Ex. and                addition                                  with lubricant    Comp. Ex.           additive        (wt. %)                                  (mg/m.sup.2)    __________________________________________________________________________    Ex. 17 poly(butena)    5      138           (TERATHANE 2900; product of           E.I. du Pont de Nemours &           Co.)    Ex. 18 poly(butena)    10     186           (TERATHANE 2900; product of           E.I. du Pont de Nemours &           Co.)    Ex. 19 poly(butena)    30     245           (TERATHANE 2900; product of           E.I. du Pont de Nemours &           Co.)    Ex. 20 ethylene oxide/propylene                           10     143           oxide copolymer           (PLURONIC L121; a product           of Asahi Denka Kogyo K.K.)    Comp.  --              0       42    Ex. 5    Comp.  liquid paraffin 5       72    Ex. 6    Comp.  polybutene(average MW: 550)                           5       65    Ex. 7    __________________________________________________________________________

The amount of addition was expressed in terms of the proportion based onthe binder. Further, the amount of impregnation with a lubricant wasexpressed in terms of the amount of deposition of a lubricant per unitsurface area of the magnetic disk.

It was confirmed that the same effect as that attained in Examples 1 and2 through the use of α-Fe₂ O₃ or γ-Fe₂ O₃ could be obtained by makinguse of α-Fe₂ O₃, SiO₂, Al₂ O₃, SiC, ZrO₂, polymer particles, etc. asnon-magnetic particles and iron powder, γ-Fe₂ O₃ containing cobalt, Fe₃O₄, Fe₃ O₄ containing cobalt, barium ferrite, etc. as magneticparticles.

Further, in the above examples, a thermosetting resin of theepoxy-phenol resin system was used as the binder for the magnetic layerand the undercoating layer, and an ultraviolet- or electron beam-curableresin was used as the binder for the undercoating layer. However, it isneedless to say that other kinds of thermosetting resins andultraviolet- or electron beam-curable resins suited for attaining thepurposes of the present invention can be used.

As is apparent from the foregoing detailed description, the presentinvention enables the production of a magnetic recording medium having anumber of fine pores running from the undercoating layer to the surfaceof the magnetic layer and the impregnation of the fine pores with alubricant in an amount sufficient to meet the impregnation requirementthrough a combination of a properly chosen thermal-decomposable additivewith a heat treatment or a combination of the addition of athermoplastic resin with an extraction treatment, which not onlycontributes to a remarkable improvement in the wear resistance but alsomakes it possible to prepare a magnetic recording medium havingexcellent durability and reliability which is suitable for use inhigh-density recording.

Further, the present invention realized the preparation of a magneticrecording medium having a number of fine pores which could not be formedby the prior art methods through the addition of a poly(alkylene oxide)to the magnetic paint, which poly(alkylene oxide) is an additiveproperly chosen while taking into consideration the compatibility withthe binder for the magnetic coating film and the thermal decomposabilitymeeting the requirement on the relationship with the curing temperatureof the binder. The magnetic recording medium thus prepared can beimpregnated with a large amount of a lubricant and exhibits reducednoises during recording and reproduction of information, which makes itpossible to realize a high-density magnetic recording medium havingexcellent durability.

What is claimed is:
 1. A method for producing a magnetic recordingmedium which comprises coating a substrate with a magnetic paintcomprising a first binder containing a thermosetting resin, magneticparticles, a solvent and an additive which can be decomposed duringthermal curing of said thermosetting resin, heating the resultingcoating film to thermally cure said thermosetting resin and, at the sametime, to bond said magnetic particles to each other, thereby forming amagnetic layer having a number of fine pores on said substrate, andimpregnating said fine pores with a lubricant, wherein said additive isa polyalkylene oxide.
 2. A method for producing a magnetic recordingmedium according to claim 1, wherein said polyalkylene oxide is at leastone compound selected from the group consisting of polybutene oxide,polypropylene oxide and a copolymer thereof, and a copolymer of ethyleneoxide with propylene oxide.
 3. A method for producing a magneticrecording medium which comprises coating a substrate with anundercoating layer forming paint containing a second binder composedmainly of at least one resin selected from the group consisting of athermosetting resin, an ultraviolet-curable resin and an electronbeam-curable resin and an easily thermal-decomposable additive having athermal decomposability greater than that of said second binder tothereby form an undercoating layer, subjecting said undercoating layerto primary curing by making use of at least one means selected from thegroup consisting of heating, ultraviolet radiation and electron beamradiation at such a temperature that said thermal-decomposable additivecan survive, coating said undercoating layer which has been subjected toprimary curing with a magnetic layer forming paint composed mainly ofmagnetic particles, a first binder containing a thermosetting resin anda solvent, thereby forming a magnetic layer, heating and curing saidmagnetic layer and said undercoating layer at a temperature higher thanthe temperature adopted in said primary curing to cause thermaldecomposition and scattering of said thermal-decomposable additivecontained in said undercoating layer during said thermal curing, therebyforming a number of fine pores in said undercoating layer and, at thesame time, forming a fine pathway running from said fine pores to thesurface of said magnetic layer, and impregnating said fine pores in saidundercoating layer with a lubricant from said fine pathway.
 4. A methodfor producing a magnetic recording medium according to claim 3, whereinsaid magnetic layer forming paint further contains an easilythermal-decomposable additive.
 5. A method for producing a magneticrecording medium according to claim 4, wherein said easilythermal-decomposable additive is a polyalkylene oxide.
 6. A method forproducing a magnetic recording medium according to claim 5, wherein saidpolyalkylene oxide is at least one compound selected from the groupconsisting of polybutene oxide, polypropylene oxide and a copolymerthereof, and a copolymer of ethylene oxide with propylene oxide.
 7. Amethod for producing a magnetic recording medium according to claim 3,wherein said easily thermal-decomposable additive is a polyalkyleneoxide.
 8. A method for producing a magnetic recording medium accordingto claim 7, wherein said polyalkylene oxide is at least one compoundselected from the group consisting of polybutene oxide, polypropyleneoxide and a copolymer thereof, and a copolymer of ethylene oxide withpropylene oxide.
 9. A method for producing a magnetic recording mediumwhich comprises coating a substrate with an undercoating layer formingpaint containing a second binder composed mainly of at least one resinselected from the group consisting of a thermosetting resin, anultraviolet-curable resin and an electron beam-curable resin and athermoplastic resin compatible with said second binder to thereby forman undercoating layer, subjecting said undercoating layer to primarycuring by making use of at least one means selected from the groupconsisting of heating, ultraviolet radiation and electron beam radiationat such a temperature that said thermoplastic resin contained in saidundercoating layer can survive, coating said undercoating layer whichhas been subjected to primary curing with a magnetic layer forming paintcomposed mainly of magnetic particles, a first binder containing athermosetting resin and a solvent, thereby forming a magnetic layer,subjecting said magnetic layer and said undercoating layer to secondarycuring at such a temperature that said thermosetting resin contained insaid magnetic layer is incompletely cured and said thermoplastic resincontained in said undercoating layer can survive, extracting andremoving said thermoplastic resin contained in said undercoating layer,employing a solvent through said incompletely cured magnetic layer,heating and curing said undercoating layer and said magnetic layer at atemperature higher than the temperature adopted in said first and secondcurings, thereby forming a number of fine pores in said undercoatinglayer and, at the same time, forming a fine pathway in said undercoatinglayer running from said fine pores to the surface of said magneticlayer, and impregnating said fine pores in said undercoating layer witha lubricant from said fine pathway.
 10. A method for producing amagnetic recording medium according to claim 9, wherein said magneticlayer forming paint further contains a thermoplastic resin compatiblewith said first binder.
 11. A method for producing a magnetic recordingmedium according to claim 10, wherein said thermoplastic resin is apoly(alkylene oxide).
 12. A method for producing a magnetic recordingmedium according to claim 11, wherein said polyalkylene oxide is atleast one compound selected from the group consisting of polybuteneoxide, polypropylene oxide and a copolymer thereof, and a copolymer ofethylene oxide with propylene oxide.
 13. A method for producing amagnetic recording medium according to claim 9, wherein saidthermoplastic resin is a polyalkylene oxide.
 14. A method for producinga magnetic recording medium according to claim 13, wherein saidpolyalkylene oxide is at least one compound selected from the groupconsisting of polybutene oxide, polypropylene oxide and a copolymerthereof, and a copolymer of ethylene oxide with propylene oxide.